On Sun, 9 Apr 2006 at 14:48:43 -0700, YD wrote:
>--- badgers <badgers@...> wrote:
>
> >...building a PCB for a circuit that I did not design.
> > It is a 7106 A/D with LCD driver to be used as a
> > thermostat with a diode and...LM7809 volt regulator.
> >
> > The circuit is based on Fig 18 on Page 13 of
> >http://www.cpdee.ufmg.br/~aguirre/datasheets/FN3082.PDF
> > <snip>
> > This will be run off of a car electrical system so
> > the 12-14 volt input is...regulated by the 9 volt linear chip.
> > <snip>
> > PS if anyone has used this chip I am open to
> > suggestions, I already have the chip, LCD and regulator.
>
><snip>
>As someone else mentioned, the 7106 is for display
>only, you'll have to use a separate control circuit,
>using the same temperature signal. Not really hard to
>do unless you're an absolute beginner.
That would be a good idea, except that the 7106 circuit chosen
uses it in its 9V battery "inputs floating w.r.t. supplies" mode.
'Analog COMMON' (q.v.) is not supply ground and is
approximately 2.8V below V+. Thus the sensor diode is
not referred to either battery rail and a fully differential input
would be required of the 'separate control circuit'.
[I just mention this in case a second non-ideal circuit is
bolted to the first: two sow's ears do not make a right ;-) ]
Now, I'm not putting down the 7106 - it's a fine and long-lived
design, it's just not easily capable of any part of the specified
application: it isn't a thermostat, its a versatile digital display.
There is adequate information in the datasheet to allow a
redesign of the input bias and reference circuit so that it
is all ground-referenced, which would then allow a simpler
parallel control circuit to be used, but consider further...
The control circuit will need an input for set-point and, hopefully,
some kind of output of the setting. How will this be done?
Will it be an analogue input such as a potentiometer,
which also gives mechanical display of its setting?
If so, you will have a thermostat with a digital temperature
display giving an impression of precision but an analogue
setting which will seem unattractively vague in comparison.
Or will it be a digital control? Unless you use something
like BCD switches to give direct mechanical feedback, you
will need a digital display. Will this be an(other) LCD display?
There is no access to the 7106 display drivers so it will need
to be a whole new display circuit - unless you do something
clever like multiplexing the preset voltage with the diode
sensor input, again quite do-able but adding a third level
of input complexity to the rescue of this circuit.
{Multiplexing the inputs would rescue the potentiometer
setting idea, so it might be worth considering.]
Were it not that the OP already has the 7106 and LCD,
I would recommend a digital approach with a PIC micro.
However, if solder is his preferred programming language
and he isn't willing to source a more appropriate chip,
I suggest some serious design and breadboarding is
required before he commits himself to a PCB layout.
Regards, LenW
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